1. Field of the Invention
This invention relates to a medium and a method for near-field optical recording which are capable of writing, reading and erasing information using an evanescent light having a beam spot size smaller than the wavelength of a source light.
2. Description of the Related Art
Various attempts for achieving high-density optical recording, such as a super-resolution system, a pit edge recording system, and a V-groove systems, have been made. However, because all these techniques rely on a heat mode recording system, the recording densities realized by these techniques have reached the possible maximum. It is a photon mode that has recently been expected to exceed the limits of the heat mode recording system. Photon mode recording is a system of using light energy per se for optical recording. Since a photon mode recording system is capable of wavelength division multiplexing and polarized light division multiplexing, a plurality of signals can be recorded in the same pit to achieve a high recording density.
In wavelength multiplexing, however, because of difficulty in obtaining a plurality of photochromic compounds having, for example, different absorption wavelengths, the recording density that could be reached is not more than about 5 times that achieved by a general heat mode recording system. The recording density reached by a polarized light multiplexing system is not more than about twice that heretofore achieved by the state-of-the-art recording systems. Photochemical hole burning is capable of achieving a multiplexity of 100 or more but requires a low temperature of 77 K or lower for writing and reading.
In order to overcome these problems, a recording system using an electrical field by a scanning tunneling microscope (STM) and electromagnetic wave irradiation (see JP-A-2-98849) and an optical recording system using an evanescent light (see E. Betzig et al, Appl. Phys. Lett., vol. 61, p. 142 (1992) have been proposed.
These systems are disadvantageous in that the electrodes for applying an electrical field or a magnet for opto-magnetic recording makes the apparatus complicated.
To solve this problem, JP-A-6-267071 discloses a rewritable optical recording method in which an evanescent light having a smaller spot size than the wavelength is used as a light source for writing, reading and erasion, and the recording layer of the optical recording medium comprises a polymer dispersion of a photochromic compound having thermal irreversibility, such as a diarylethene derivative represented by formula: 
According to the method disclosed, high-density recording can be accomplished without using electrodes or a magnet.
In order to effect optical recording with an evanescent light without using electrodes or a magnet, it is necessary for the recording layer itself to show reversible changes between two states accompanied by color changes by a light source with no other physical or chemical perturbation and have thermal stability in each state. JP-A-6-267071 have proposed such a recording layer and succeeded in forming recorded pits whose size is {fraction (1/10)} to {fraction (1/100)} as large as the pit formed in the state-of-the-art optical recording media by using an evanescent light without using electrodes nor a magnet, thereby achieving 100- to 10,000-fold increased recording density.
In order to increase the recording density of optical recording media, it is important, above all, that the recording layer should undergo a large change in its state, for example, absorbance or refractive index upon irradiation. In using a photochromic compound, it is also of extreme importance that the molecules of the compound be present in the recording layer at an increased density. Further, decreased roughness of the recording layer surface will minimize scattering of the light thereby to improve the recording density.
Small surface smoothness of the recording layer is desirable particularly in writing and reading information with an evanescent light because the distance between an evanescent light source and the recording layer should be as small as ¼ or less of the wavelength of the light.
However, considerations for recording density improvement from these aspects are not given in JP-A-6-267071. That is, the proposed recording layer contains a binder to secure transparency (amorphous property) in a thin film configuration. As a result, the photochromic compound is about 20-fold diluted only to have a low molar absorptivity (ε), which is problematical for practical use.
JP-A-9-241254 discloses a diheteroarylethene compound having an adamantyl group which is represented by formula: and an optical recording medium containing the same. The disclosure teaches that the compound, when dissolved in a solvent, applied to a substrate and dried, provides an amorphous recording layer without using a binder resin. However, the glass transition point of this compound is as low as 40° C. A recording layer formed of such a compound involves the fear that the pits after recording may change the phase under a high temperature and high humidity condition. It will follow that the signals become unclear, resulting in a failure of accurate read-out.
Jpn. J. Appl. Phys., Part 1, 35(3) (1966) reports a recording medium having a polystyrene resin recording layer containing 20% by weight of a diheteroarylethene compound represented by formula: 
The report also discloses a recording medium having an amorphous thin film as a recording layer which is formed on a glass substrate by vacuum evaporation of a diheteroarylethene compound represented by formula: 
The former recording medium cannot be seen as practical due to the low concentration of the heteroarylethene compound in the recording layer similarly to the one according to JP-A-6-267071. Having a glass transition point of Compound A is 18° C. and that of Compound B is 25° C. the latter diheteroarylethene compound has the same problem as with JP-A-9-241254. Besides, while the deposited film is uniformly amorphous immediately after vacuum evaporation, crystal nuclei appear in a day or two, and gradual crystallization of the film follows. Therefore, these recording media need further improvements for practical use.
An additional number of optical recording media having a recording layer containing a diheteroarylethene compound are found in JP-A-62-273231, JP-A-7-89954, JP-A-9-58188, JP-A-7-173151, etc. The recording layers according to JP-A-7-89954, JP-A-9-58188, and JP-A-7-173151 comprise a binder resin in a large proportion so as to form an amorphous film. Because the diheteroarylethene compound of JP-A-62-273231 is a polymer, the recording layer can be made solely of this compound and yet cannot get rid of the problem of small molar absorptivity.